Pseudoplastic powdered lacquer slurry free of organic solvents method for production and the use thereof

ABSTRACT

The invention relates to a pseudoplastic, powdered lacquer slurry, free of organic solvents, comprising solid and/or highly viscous particles, with an average particle size of 0.8 to 20 μm, which are dimensionally stable under conditions of storage and usage, whereby at least 99% of the particles have a particle size ≦30 μm. Said slurry may be produced by: 1) emulsifying an organic solution, containing binders and cross-linking agents, resulting in an oil-in-water type emulsion, 2) removing the organic solvent(s) and 3) wet milling the resulting slurry. The invention further relates to the use of the powdered lacquer slurry, for the production of lacquers for automobile production, automobile repair work, furniture painting and industrial painting including coil coating, container coating and the coating of electrical components.

[0001] The present invention relates to a novel powder clearcoat slurry,free from organic solvents, which possesses pseudoplasticity. Thepresent invention also relates to a novel process for preparing thispowder clearcoat slurry. The invention relates not least to the use ofthe novel powder clearcoat slurry for producing clearcoats for theautomotive sector and for the industrial sector.

[0002] For the coating of automobile bodies, preference is given todayto the use of liquid coating materials, i.e., spray paints. These giverise to numerous environmental problems owing to their solvent content.The same applies to cases where waterborne clearcoat materials are used,since they still always contain certain amounts of organic solvents.

[0003] Waterborne clearcoat materials of this kind are known from theGerman patent DE 196 23 371 A1. Directly after application, theconventional waterborne clearcoat materials do not dry to a powder butinstead flow out to form a continuous film. They comprise aqueoussecondary dispersions and are used in the automotive sector for aqueousmulticoat systems or aqueous one-component or two-component clearcoats.The aim here is for sedimentation-stable dispersions having an averageparticle size of from about 10 to about 200 nm The reason for this isthe experience, familiar to the skilled worker, whereby, the better thestabilization and the smaller the size of dispersion particles, the lesstheir tendency to settle. For reliable application characteristics andin order to reduce the popping tendency, however, it is necessary to useup to 20% by weight of solvents as well.

[0004] For this reason, increased efforts have been made in recent yearsto use powder coating materials for the coating. The results to date,however, have not been satisfactory; in particular, powder clearcoatmaterials still show weaknesses with regard to chemical resistance andyellowing.

[0005] In the meantime, many developments have aimed to provide powderclearcoat materials in the form of aqueous dispersions that can beprocessed using liquid coating technologies. These dispersions are alsoreferred to as powder clearcoat slurries. These customary and knownpowder clearcoat slurries, in contradistinction to the powder clearcoatmaterials, may be processed in conventional wet coating plants and maybe applied at substantially lower coat thicknesses of about 40 μm,relative to about 80 μm in the case of powder coating materials, withgood leveling and with a chemical resistance which is comparable withthat of the powder coating materials.

[0006] For instance, the European patent EP 0 652 264 A1 or the Germanpatent application DE 196 18 657 A1 discloses a powder clearcoat slurryin which the solid binder and crosslinker components and, if desired,additives are, as is usual for the production of powder coatingmaterials, first of all coextruded and then subjected to dry grinding,after which, in a further step of wet grinding, they are converted intoa powder clearcoat slurry with the aid of emulsifiers and wettingagents.

[0007] In order that in the course of wet grinding there is no cloggingof the grinding equipment—the stirred ball mills, for example—the powderclearcoat slurries or the particles present therein are required inaccordance with the aforementioned German patent application to have aglass transition temperature of from 20 to 90° C., preferably from 40 to70° C. This boundary condition considerably restricts the scope forvariation of the material composition of the known powder clearcoatslurries. And yet it is not possible to do without wet grinding, sinceotherwise the coarse particle fraction of the powder clearcoat slurry isnot reduced in size.

[0008] The known clearcoat powder slurry must on the one hand, onperformance grounds, contain comparatively coarse particles with anaverage size of from 2 to 6 μm. On the other hand, it has to be filteredprior to application. In the course of this filtering operation, evenvery small coarse particle fractions (particle size >10 μm) result inclogging of the filters, so that the production process has to bestopped—a disadvantage both technically and economically.

[0009] The German patent DE 196 17 086 A1 discloses a powder clearcoatslurry in which the average size of the solid particles is from 0.1 to10 μm. It is preferred in this case to employ average particle sizes offrom 0.23 to 0.43 μm. For stabilization, it is necessary, in addition tothe ionic stabilization, to employ external emulsifiers aswell—generally polyethylene oxide adducts, which decrease the resistanceof the coating to water and moisture. Moreover, these known powderclearcoat slurries still always include certain amounts of organiccosolvents or leveling agents, which cannot be removed since they areessential to the leveling properties of the partly dried film. Moreover,special equipment such as pressure release homogenizing nozzles arenecessary for their preparation. Prior to their application, they areadjusted to the application viscosity using thickeners. A complexviscosity behavior is not described. Furthermore, the patent does notgive any teaching as to how the problem of filterability in connectionwith powder clearcoat slurries might be solved.

[0010] The German patent application DE 198 41 842.6, unpublished at thepriority date of the present specification, describes a powder clearcoatslurry which is free from organic solvents and external emulsifiers andcomprises solid spherical particles with an average size of from 0.8 to20 μm and a maximum size of 30 μm, the powder clearcoat slurry having anion-forming group content of from 0.05 to 1 meq/g, a neutralizing agentcontent of from 0.05 to 1 meq/g, and a viscosity of (i) from 50 to 1000mPas at a shear rate of 1 000 s⁻¹, (ii) from 150 to 8000 mPas at a shearrate of 10 s^(−1,) and (iii) from 180 to 12 000 mPas at a shear rate of1 s⁻¹.

[0011] This powder clearcoat slurry can be prepared with a smallernumber of processing steps than the known powder clearcoat slurries;owing to its typical powder slurry properties, however, with residualsolvent contents of <1%, and its comparable particle sizes, it hasapplication characteristics similarly advantageous to the knownslurries. In contrast to the known waterborne clearcoat materials, ithas reliable application characteristics in terms of popping marks atthe required film thicknesses of about 40-50 μm even without the aid oforganic solvents. Moreover, it and the process for its preparationcontinue to exhibit the main advantage of the mixing of components insolution, namely the very good homogeneity of the resulting particles.It has been found that even this powder clearcoat slurry must befiltered prior to its application and in the course of such filtrationgives rise to problems similar to those encountered with the knownpowder clearcoat slurries. Whether and, if so, under what boundaryconditions the powder clearcoat slurry described in DE-A-198 41 842.6can be subjected to wet grinding is not stated therein.

[0012] It is an object of the present invention to provide a novelpowder clearcoat slurry which no longer has the disadvantages of theprior art. In particular, the novel powder clearcoat slurry should bepreparable with a smaller number of processing steps than theconventional powder clearcoat slurries. However, on the basis of itstypical powder slurry properties, with residual solvent contents of <1%,and its comparable particle sizes, it should have similarly advantageousapplication characteristics to said conventional powder clearcoatslurries. In contrast to the known waterborne clearcoat materials, thenovel powder clearcoat slurries should ensure reliable applicationcharacteristics with regard to popping marks at the required filmthicknesses of approximately 40-50 μm, even without the assistance oforganic solvents. Moreover, they should be able to be subjected to wetgrinding without any problems. In this context their materialcomposition ought to be able to be varied with considerably greaterwidth than that of the known powder clearcoat slurries, without anyproblems occurring during wet grinding. In other words, the novel powderclearcoat slurry ought to have a considerably wider processing window interms of wet grinding than the known slurries. After wet grinding, thenovel powder clearcoat slurry should be filterable without problems.

[0013] A further object of the present invention was to find a novelprocess for preparing powder clearcoat slurries which continues toensure the essential advantage of the mixing of the components insolution: the very good homogeneity of the resulting particles.

[0014] The invention accordingly provides the novel, pseudoplasticpowder clearcoat slurry which is free from organic solvents andcomprises solid and/or highly viscous, particles which are dimensionallystable under storage and application conditions and have an average sizeof from 0.8 to 20 μm, at least 99% of the particles having a size ≦30μm, which is preparable by

[0015] 1) emulsifying an organic solution comprising binder andcrosslinking agent to give an emulsion of the oil-in-water type,

[0016] 2) removing the organic solvent or solvents, and

[0017] 3) wet grinding the resulting slurry.

[0018] In the text below, the novel, pseudoplastic powder clearcoatslurry free from organic solvents is referred to for short as the“slurry of the invention”.

[0019] Further subject matter of the invention will emerge from thedescription.

[0020] In the light of the prior art it was surprising and unforeseeablefor the skilled worker that the object on which the present invention isbased could be achieved by means of the slurry of the invention. Aparticular surprise was that the slurry of the invention could bewet-ground at temperatures which lay above the minimum film-formingtemperature (MFFT) of its particles without any problems occurring. Afurther surprise was that the slurry of the invention was stable evenwithout external emulsifiers.

[0021] For the slurry of the invention it is essential for the averagesize of the solid particles to be from 0.8 to 20 μm, and particularlypreferably from 2 to 6 μm. The average particle size is understood asmeaning the 50% median determined according to the laser diffractionmethod, i.e., 50% of the particles have a diameter≦the median and 50% ofthe particles have a diameter≧the median.

[0022] Slurries having average particle sizes of this kind and a solventcontent of <1% exhibit better application characteristics and, at theapplied film thicknesses of >30 μm as presently practiced in theautomotive industry for the finishing of automobiles, exhibit much lessof a tendency toward popping and “mudcracking” than conventional powderclearcoat slurries.

[0023] The upper limit of particle size is reached when the size of theparticles means that they are no longer able to flow out fully onbaking, and thus the film leveling is adversely affected. In cases whererequirements regarding the appearance are not very stringent, however,the limit may also be higher. 30 μm is considered a sensible upperlimit, since above this particle size the rinse ducts of the highlysensitive application apparatus are likely to become blocked.

[0024] The particles in the slurry of the invention are solid and/orhighly viscous. In the context of the present invention, “highlyviscous” means that the particles behave essentially like solidparticles under the customary and known conditions of the storage andapplication of powder clearcoat slurries.

[0025] The particles in the slurry of the invention are alsodimensionally stable. In the context of the present invention,“dimensionally stable” means that the particles, under the customary andknown conditions of the storage and application of powder clearcoatslurries, neither agglomerate nor breakdown into smaller particles butinstead essentially retain their original form even under the influenceof shear forces.

[0026] The slurry of the invention is free from organic solvents. In thecontext of the present invention this means that it has a residualvolatile solvent content of <1% by weight, preferably <0.5% by weight,and with particular preference <0.2% by weight. In accordance with theinvention it is of very particular advantage if the residual content isbelow the gas-chromatographic detection limit.

[0027] The above-described particle sizes for use in accordance with theinvention are obtained even without the aid of additional externalemulsifiers if the binder has an ion-forming group content correspondingto an average acid number or amine number of from 3 to 56 g KOH/g solids(MEQ acid or MEQ amine of from 0.05 to 1.0 meq/g solids), preferably upto 28 (MEQ acid or MEQ amine: up to 0.5 meq/g solids), and in particularup to 17 (MEQ acid or MEQ amine: up to 0.3 meq/g solids).

[0028] In accordance with the invention, the general aim is for a lowamount of such groups, since, when the customary crosslinking agents areused, such as blocked polyisocyanates, for example, free groups of thiskind may remain in the film and may reduce the strength with regard toenvironmental substances and chemicals. On the other hand, the acidgroup content must still be sufficiently high to ensure the desiredstabilization.

[0029] The ion-forming groups are neutralized 100%, or else onlypartially neutralized to <100%, with the aid of neutralizing agents. Theamount of neutralizing agent is chosen such that the MEQ value of theslurry of the invention is below 1, preferably below 0.5, and inparticular below 0.3 meq/g solids. In accordance with the invention itis of advantage if the amount of neutralizing agent corresponds at leastto an MEQ value of 0.05 meq/g solids.

[0030] In general, therefore, the chemical nature of the binder is notrestrictive provided it comprises ion-forming groups which areconvertible by neutralization into salt groups and so are able to takeon the function of ionically stabilizing the particles in water.

[0031] Suitable anion-forming groups are preferably acid groups such ascarboxylic, sulfonic or phosphonic acid groups. Accordingly, theneutralizing agents used are bases, such as alkali metal hydroxides,ammonia, or amines. Alkali metal hydroxides are suitable for use only toa limited extent, since the alkali metal ions are nonvolatile on bakingand, owing to their incompatibility with organic substances, may cloudthe film and lead to instances of loss of gloss. Consequently, ammoniaor amines are preferred. In the case of amines, preference is given towater-soluble tertiary amines. By way of example, mention may be made ofN,N-dimethylethanolamine or aminomethylpropanolamine (AMP).

[0032] Suitable cation-forming groups are preferably primary, secondaryor tertiary amines. Accordingly, neutralizing agents used are, inparticular, low molecular mass organic acids such as formic acid, aceticacid or lactic acid.

[0033] Binders which contain cation-forming groups are known from thefield of electrodeposition coating materials. By way of example,reference may be made to the patents EP-A-0 012 463, EP-A-0 612 818 orU.S. Pat. No. 4,071,428.

[0034] For the preferred use of the slurry of the invention asunpigmented clearcoat materials in automotive finishing, preference isgiven to polymers or oligomers containing acid groups as ion-forminggroups, since these so-called anionic binders are generally moreresistant to yellowing than the class of the cationic binders.

[0035] Nevertheless, cationic binders with groups convertible intocations, such as amino groups, are likewise suitable for use inprinciple provided the field of use is tolerant of their typicalsecondary properties, such as their tendency toward yellowing.

[0036] As binders which contain anion-forming groups, it is possible touse any desired resins containing the abovementioned acid groups.However, it is important that they also carry further groups whichensure crosslinkability. In accordance with the invention, hydroxylgroups are preferred.

[0037] Suitable oligomers and polymers of this kind for use inaccordance with the invention include hydroxyl-containing, preferablylinear and/or branched and/or block, comb and/or randompoly(meth)acrylates, polyesters, alkyds, polyurethanes, acrylatedpolyurethanes, acrylated polyesters, polylactones, polycarbonates,polyethers, (meth)acrylatediols or polyureas.

[0038] Besides the hydroxyl groups, the oligomers and polymers may alsoinclude other functional groups such as acryloyl, ether, amide, imide,urethane, urea, thio, carbonate or epoxide groups, provided these do notdisrupt the crosslinking reactions.

[0039] These oligomers and polymers are known to the skilled worker, andmany suitable products are available on the market.

[0040] In accordance with the invention, the polyacrylates, thepolyesters, the alkyd resins, the polyurethanes and/or the acrylatedpolyurethanes are of advantage and are therefore used with preference.

[0041] Examples of suitable polyacrylates are described in the Europeanpatent application EP-A-0 767 185 and the American patents U.S. Pat.Nos. 5,480,493, 5,475,073 or 5,534,598. Further examples of particularlypreferred polyacrylates are sold under the brand name Joncryl^(R), suchas Joncryl^(R) SCX 912 and 922.5, for instance. The preparation of thesepolyacrylates is widely known and is described, for example, in thestandard work Houben-Weyl, Methoden der organischen Chemie, 4th edition,volume 14/1, pages 24 to 255, 1961.

[0042] The preparation of the polyesters and alkyd resins which are usedwith preference in accordance with the invention is widely known and isdescribed, for example, in the standard work Ullmanns Encyklopadie dertechnischen Chemie, 3rd edition, volume 14, Urban & Schwarzenberg,Munich, Berlin, 1963, pages 80 to 89 and pages 99 to 105, and also inthe following books: “Résines Alkydes-Polyesters” by J. Bourry, Paris,Dunod, 1952, “Alkyd Resins” by C. R. Martens, Reinhold PublishingCorporation, New York, 1961, and “Alkyd Resin Technology” by T. C.Patton, Interscience Publishers, 1962.

[0043] The polyurethanes and/or acrylated poly-urethanes whose use isparticularly preferred in accordance with the invention are described,for example, in the patents EP-A-0 708 788, DE-A-44 01 544 or DE-A-19534 361.

[0044] Suit,able crosslinkers include all crosslinking agents which arecustomary in the field of light-stable clearcoat materials. Examplesthereof are etherified melamine-formaldehyde resins, benzoguanamineresins, resins or compounds containing siloxane groups, resins orcompounds containing anhydride groups, resins or compounds containingepoxide groups, blocked and/or unblocked polyisocyanates, and/ortris(alkoxycarbonylamino)triazines, as described in the patents U.S.Pat. Nos. 4,939,213, 5,084,541, 5,288,865 or EP-A-0 604 922. Inaccordance with the invention, the blocked polyisocyanates are ofadvantage and are therefore used with particular preference. Examples ofsuitable blocked polyisocyanates are described in the German patentsDE-A-196 17 086 and 196 31 269 and also in the European patents EP-A-0004 571 and 0 582 051.

[0045] The slurry of the invention comprises preferably nonionic andionic thickeners. This effectively counters the tendency of thecomparatively large solid particles toward sedimentation, and at thesame time pseudoplasticity is brought about.

[0046] Examples of nonionic thickeners are hydroxyethylcellulose andpolyvinyl alcohols. Nonionic associative thickeners are likewiseavailable on the market in diverse selection. They consist for exampleof water-dilutable polyurethanes, which are the reaction products ofwater-soluble polyetherdiols, aliphatic diisocyanates and monofunctionalhydroxy compounds containing an organophilic radical.

[0047] Likewise commercially available are ionic thickeners. Theseusually contain anionic groups and are based for example on specialpolyacrylate resins containing acid groups, some or all of which mayhave been neutralized.

[0048] Examples of suitable thickeners for use in accordance with theinvention are known from the text-book “Lackadditive” [Additives forcoatings] by Johan Bieleman, Wiley-VCH, Weinheim, N.Y., 1998, pages 31to 65.

[0049] For the slurry of the invention it is especially advantageous ifboth of the above-described types of thickener are present therein. Theamount of thickeners to be added and the ratio of ionic to nonionicthickener is guided by the desired viscosity of the slurry of theinvention, which in turn is determined by the required sedimentationstability and by the special requirements of spray application. Theskilled worker will therefore be able to determine the amount of thethickeners and the ratio of the thickener types to one another on thebasis of simple considerations, possibly with the aid of preliminarytests.

[0050] Preferably, a viscosity range of from 50 to 1500 mPas at a shearrate of 1000 s⁻¹ and from 150 to 8000 mPas at a shear rate of 10 s⁻¹ isset.

[0051] This viscosity behavior, known as “pseudoplasticity”, describes astate which does justice both to the requirements of spray application,on the one hand, and to the requirements in terms of storage andsedimentation stability, on the other: in the state of motion, such aswhen pumping the slurry of the invention in circulation in the ringcircuit of the coating installation and when spraying, for example, theslurry of the invention adopts a state of low viscosity which ensureseasy processability. Without shear stress, on the other hand, theviscosity rises and thus ensures that the coating material alreadypresent on the substrate to be coated has a reduced tendency to formruns on vertical surfaces. In the same way, a result of the higherviscosity in the stationary state, such as during storage, for instance,is that sedimentation of the solid particles is largely prevented orthat any slight degree of settling of the powder slurry of the inventionduring the storage period can be removed again by agitation.

[0052] In addition to the essential constituents described above, thesolid particles of the slurry of the invention may comprise additivessuch as are commonly used in clearcoat materials. In this context it isessential that these additives do not substantially lower the minimumfilm-forming temperature (MFFT) or the glass transition temperature Tgof the binders.

[0053] Examples of suitable additives are polymers, crosslinkingcatalysts, defoamers, adhesion promoters, additives for improvingsubstrate wetting, additives for improving surface smoothness, flattingagents, light stabilizers, corrosion inhibitors, biocides, flameretardants, and polymerization inhibitors, especially photoinhibitors,as described in the book “Lackadditive” by Johan Bielemann, Wiley-VCH,Weinheim, N.Y., 1998.

[0054] Crosslinking components of polyol type, reactive diluents orleveling assistants which may be incorporated by crosslinking in thefilm may be added to the slurry of the invention. It is important,however, that these components are located preferably in the external,aqueous phase of the slurry of the invention and not in the disperseorganic phase, where they would bring about a lowering of the MFFT ofthe particles contained herein and thus coalescence or coagulation ofany sedimented particles.

[0055] Examples of suitable compounds of this kind are oligomericpolyols, which are obtainable by hydroformylation and subsequenthydrogenation from oligomeric intermediates themselves obtained bymetathesis reactions of acyclic monoolefins and cyclic monoolefins;examples of suitable cyclic monoolefins are cyclobutene, cyclopentene,cyclohexene, cyclooctene, cycloheptene, norbornene or 7-oxanorbornene;examples of suitable acyclic monoolefins are present in hydrocarbonmixtures which are obtained in petroleum processing by cracking (C₅cut); examples of suitable oligomeric polyols for use in accordance withthe invention have a hydroxyl number (OHN) of from 200 to 450, anumber-average molecular weight Mn of from 400 to 1000D, and amass-average molecular weight M_(w) of from 600 to 1100; furtherexamples of suitable compounds of this kind are branched, cyclic and/oracyclic C₉-C₁₆ alkanes functionalized with at least two hydroxyl groups,especially positionally isomeric diethyloctanediols, and alsocyclohexanedimethanol, neopentyl glycol hydroxypivalate, neopentylglycol, trimethylolpropane or pentaerythritol.

[0056] It is of advantage in accordance with the invention to preparethe slurry of the invention by means of the process of the invention.

[0057] In the process of the invention, the ionically stabilizablebinders and the crosslinking agents and also, if appropriate, theadditives are mixed in organic solution and dispersed together in waterwith the aid of neutralizing agents by the secondary dispersion process.The system is then, diluted with water, while stirring. A water-in-oilemulsion is formed first of all, which on further dilution changes tobecome an oil-in-water emulsion. This point is generally reached atsolids contents of <50% by weight, based on the emulsion, and is evidentexternally from a relatively sharp drop in viscosity in the course ofdilution.

[0058] The emulsion thus obtained, which still contains solvent, issubsequently freed from solvents (stripping) by means of azeotropicdistillation.

[0059] In accordance with the invention it is of advantage if theminimum film-forming temperature of the binders is at least 0° C.,preferably at least 10° C., with particular preference at least 15° C.,with very particular preference at least 20° C., and in particular atleast 25° C. The minimum film-forming temperature may be determined bydrawing down the dispersion onto a glass plate using a bar coater andheating it in a gradient oven. The temperature at which the pulverulentlayer films is designated the minimum film-forming temperature. Forfurther details, refer to Römpp Lexikon Lacke und Druckfarben, GeorgTheime Verlag, Stuttgart, New York, 1998 “minimum film-formingtemperature”, page 391.

[0060] It is of advantage in accordance with the invention if thesolvents to be removed are distilled off at a distillation temperaturebelow 70° C., preferably below 50° C. and in particular below 40° C. Ifappropriate, the distillation pressure is chosen so that in the case ofhigher-boiling solvents this temperature range is still maintained.

[0061] At its simplest, the azeotropic distillation may be realized bystirring the emulsion at room temperature in an open vessel for severaldays. In the preferred case, the solvent-containing emulsion is freedfrom the solvents by a vacuum distillation.

[0062] In order to avoid high viscosities, the amount of water andsolvents removed by distillation or evaporation is preferably replacedby water. The water may be added before, during and/or after, preferablybefore, the evaporation or distillation, in portions.

[0063] After the solvents have been lost, the MFFT of the dispersedparticles rises, and instead of the previous solvent-containing emulsion(liquid-in-liquid dispersion) a solid-in-liquid dispersion is formed. Inthe context of the present invention, the concept of a solid-in-liquiddispersion also embraces a dispersion of highly viscous particles inwater.

[0064] In accordance with the invention, the particles of the resultingslurry are mechanically comminuted in the wet state, this being referredto in the context of the present invention as wet grinding of theslurry. In this grinding process, conditions are employed such that thetemperature of the ground material does not exceed 70° C., preferably60° C., and in particular 50° C. The specific energy input during thegrinding process is preferably 10 to 1000, more preferably from 15 to750, and in particular from 20 to 500 Wh/g.

[0065] Wet grinding can be carried out using any of a wide variety ofapparatus which produces high or low shear fields.

[0066] Examples of suitable apparatus which produces low shear fieldsincludes customary and known stirred tanks, slot homogenizers,microfluidizers or dissolvers.

[0067] Examples of suitable apparatus which produces high shear fieldsinclude customary and known stirred mills or inline dissolvers

[0068] In accordance with the invention it is particularly preferred toemploy the apparatus which produces high shear fields. Among suchapparatus, the stirred mills are particularly advantageous in accordancewith the invention and are therefore used with very particularpreference.

[0069] In general, in the course of wet grinding, the slurry is suppliedto the apparatus described above by means of suitable equipment such aspumps, and is circulated through said apparatus until the desiredparticle size has been reached and the slurry of the invention isobtained.

[0070] In accordance with the invention it is of advantage in particularfor energy reasons if the slurry to be ground contains onlysome—preferably from 5 to 90% by weight, more preferably from 10 to 80%by weight, and in particular from 20 to 70% by weight—of theabove-described thickeners present in the slurry of the invention. Wherethis variant of the process of the invention is employed, the remainingamount of thickener is to be added after wet grinding, so giving theslurry of the invention.

[0071] The slurry of the invention advantageously has a solids contentof from 10 to 60% by weight, in particular from 20 to 50% by weight.

[0072] Preferably, the slurry of the invention is filtered before beingused. For this purpose use is made of the customary and known filtrationapparatus and filters, such as are also suitable for the filtration ofthe known powder clearcoat slurries. The mesh size of the filters mayvary widely and is guided primarily by the particle size and sizedistribution of the particles of the slurry of the invention. Theskilled worker is therefore able to determine the appropriate filterswith ease on the basis of this physical parameter. Examples of suitablefilters include bag filters. These bag filters are available in themarket under the brand name Pong® or Cuno®. It is preferred to use bagfilters having mesh sizes of from 10 to 50 μm, examples being Pong® 10to Pong® 50.

[0073] In the course of filtration the further particular advantage ofthe slurry of the invention becomes evident, namely that it can befiltered without problems even when in the course of wet grinding theminimum film-forming temperature of the particles present therein wasexceeded.

[0074] To produce the clearcoats of the invention, the slurry of theinvention is applied to the substrate that is to be coated. No specialmeasures need be taken here; instead, the application may take place inaccordance with the customary and known techniques, which is anotherparticular advantage of the slurry of the invention.

[0075] Following its application, the slurry of the invention drieswithout problems and does not film at the processing temperature,generally at room temperature. In other words, the slurry of theinvention applied as a wet film loses water when flashed off at roomtemperature or slightly elevated temperatures, without the particlespresent therein altering their original solid form. The solid film inpowder form loses the residual water by evaporation more easily than aflowing wet film. As a result, the risk of bubbles of evaporated waterenclosed in the cured film (popping) is reduced. Moreover, the tendencytoward mudcracking is extremely low.

[0076] In the subsequent baking step, the now substantially water-freepowder layer is melted and caused to crosslink. In some cases, it may beof advantage to carry out the leveling process and the crosslinkingreaction with a chronological offset, by operating in accordance with astaged heating program or a so-called heating ramp. The appropriatecrosslinking temperature for the present examples is between 120 and160° C. The corresponding baking time is between 20 and 60 minutes.

[0077] The clearcoat which results in this case has outstandingperformance properties. For instance, it adheres firmly to all customaryand known basecoats or to substrates such as metal, glass, wood orplastic. It is of high gloss, smooth, scratch-resistant, stable toweathering and free from defects. On the basis of its advantageousprofile of properties, moreover, the slurry of the invention is alsosuitable for applications other than automotive finishing, especiallyfor the coating of furniture and for industrial coating, including coilcoating and container coating, and the coating of electrical components.

EXAMPLE Preparation Example 1 The Preparation of a Solution PolyacrylateResin

[0078] 445.3 parts by weight of methyl ethyl ketone (MEK) wereintroduced into a reaction vessel and heated to 80° C. The initiatorsolution, consisting of 47.6 parts by weight of TBPEH (tert-butylperethylhexanoate) and 31.1 parts by weight of MEK, and the monomermixture, consisting of 183.3 parts by weight of tert-butyl acrylate,71.4 parts by weight of n-butyl methacrylate, 95.2 parts by weight ofcyclohexyl methacrylate, 121.4 parts by weight of hydroxyethylmethacrylate and 4.76 parts by weight of acrylic acid, were metered intothis initial charge at 80° C. with stirring over the course of 5 h fromtwo separate feed vessels. The reaction mixture was then heated to 80°C. for two hours, and a fraction of the volatile components of thereaction mixture was stripped off under reduced pressure at 500 mbar for5 h. The resin solution was then cooled to 50° C. and discharged.

[0079] The resin solution had the following characteristics:

[0080] Solids: 71.4% by weight (1 h at 130° C.)

[0081] Viscosity: 8.0 dPas (cone and plate viscometer at 23° C.; 55%strength solution)

[0082] Acid number: 10.1 mg KOH/g resin solids

Preparation Example 2 The Preparation of a Blocked PolyisocyanateCrosslinker

[0083] 837 parts of isophorone diisocyanate were introduced into anappropriate reaction vessel, and 0.1 part of dibutyltin dilaurate wasadded. A solution of 168 parts of trimethylolpropane and 431 parts ofmethyl ethyl ketone was then run in slowly. As a result of theexothermic reaction, the temperature rose. After it had reached 80° C.,the temperature was kept constant by external cooling and the rate ofaddition of the feed stream was reduced slightly if necessary. After theend of the feed stream, the mixture was held at this temperature forabout 1 hour until the isocyanate content of the solids had reached15.7% by weight based on the solids. The reaction mixture wassubsequently cooled to 40° C. and a solution of 362 parts of3,5-dimethylpyrazole in 155 parts of methyl ethyl ketone was added overthe course of 30 minutes. After the reaction mixture had heated up to80° C., owing to the exothermic reaction, the temperature was keptconstant for 30 minutes until the NCO content had dropped to less than0.1% by weight. Then 47 parts of n-butanol were added to the reactionmixture, which was held at 80° C. for a further 30 minutes and then,after brief cooling, was discharged.

[0084] The reaction product had a solids content of 70% by weight (1 hat 130° C.).

Example 1 The Preparation of an Inventive Powder Clearcoat Slurry

[0085] 975.1 parts by weight of the acrylate resin solution frompreparation example 1 and 567.7 parts by weight of the crosslinkersolution from preparation example 2 were mixed at room temperature in anopen vessel for 15 minutes with stirring. Then 10.9 parts by weight ofCyagard® 1164 L (UV absorber from Cytec), 10.9 parts by weight ofTinuvin® liquid 123 (sterically hindered amine “HALS” from Ciba Geigy)and 9.5 parts by weight of N,N-dimethylethanolamine were added and theresulting mixture was stirred at room temperature for a further twohours. The mixture was then diluted with 645.9 parts by weight ofdeionized water in small portions. After an interval of 15 minutes, afurther 780.0 parts by weight of water were added. This gave an aqueousemulsion of low viscosity with a theoretical solids content of 37% byweight, which was stirred at room temperature for a further 48 hours.The amount of liquid evaporated off was supplemented by adding deionizedwater until the original level was regained. This gave a powderclearcoat slurry having the following characteristics: Solids (2 h, 80°C.): 36.7% by weight Solvent content: <0.05% (by gas chromatography)

[0086] In order to produce the desired pseudo-plasticity, 22.6 parts byweight of Acrysol® RM 8 (nonionic associative thickener from Rohm &Haas) and 6.0 parts of Viskalex® HV 30 (anionic thickener based onpolyacrylate resin, from Allied Colloids) were incorporated by stirringinto 1000 parts by weight of this powder clearcoat slurry. The viscosityprofile of the resultant inventive powder clearcoat slurry 1 was asfollows:

[0087] 820 mPas at a shear rate of 100 s⁻¹

[0088] 210 mPas at a shear rate of 1000 s⁻¹

[0089] The resultant powder clearcoat slurry had a minimum film-formingtemperature of 45° C. It was ground by means of a stirred ball mill. Forthis purpose the powder clearcoat slurry, using a pump, was supplied tothe mill and circulated through the mill until the desired quality[particle sizes: x₅₀ 2.5 μm; X_(max)<10 μm (oversize: laser diffractionmeasuring instrument from Malvern)] had been reached.

[0090] The wet-ground powder clearcoat slurry, i.e., the slurry of theinvention, was filtered without problems using bag filters (Pong® 50)without any clogging of the filters.

[0091] After storage at room temperature for 4 weeks, there was a veryslight sediment, which had only settled loosely, and which could bereagitated again to homogeneity within 5 minutes using a simplelaboratory stirrer.

Example 2 The Use of the Inventive Powder Clearcoat Slurry as Part of aMulticoat Color or Effect Paint System to Produce a Clearcoat

[0092] The inventive powder clearcoat slurry of example 1 was appliedwith preparation of a so-called integrated system, which is describedbelow for a gray metallic shade.

[0093] Using a gravity feed gun, a functional coat (Ecoprime® from BASFCoatings AG) was applied to steel panels coated cathodically with acommercial electrocoat material. After flashing off at room temperaturefor 5 minutes, a gray aqueous metallic basecoat (Ecostar® from BASFCoatings AG) was applied in the same way to this coat and wassubsequently predried at 80° C. for 5 minutes.

[0094] After the panels had been cooled, the inventive powder clearcoatslurry was applied in the same way. Thereafter, the panels were firstflashed off for 5 minutes and then predried at 40° C. for 15 minutes.They were subsequently baked at 145° C. for 30 minutes.

[0095] This gave an aqueous metallic overall paint system in a grayshade. The applied wet films had been chosen so that, after baking, thedry film thicknesses for the functional coat and for the aqueousmetallic basecoat were each 15 μm. The inventive clearcoat-had a filmthickness of from 40 to 45 μm. The inventive multicoat paint system hadan outstanding overall appearance. At the applied clearcoat thickness,no defects in the form of popping marks and mudcracking are evident inthe clearcoat.

[0096] The table below gives an overview of the tests conducted and theresults obtained in them. TABLE The performance properties of theinventive clearcoat Properties Example 2 Clearcoat 40-45 μm thicknessGloss at 20°*⁾ 77 Haze*⁾ 80 Appearance bright Leveling very good Poppingmarks none Mudcracking none

What is claimed is:
 1. A pseudoplastic powder clearcoat slurry which isfree from organic solvents and comprises solid and/or highly viscousparticles which are dimensionally stable under storage and applicationconditions and have an average size of from 0.8 to 20 μm, at least 99%of the particles having a size≦30 μm, which is preparable by 1)emulsifying an organic solution comprising binder and crosslinking agentto give an emulsion of the oil-in-water type, 2) removing the organicsolvent or solvents, and 3) wet grinding the resulting slurry.
 2. Theslurry as claimed in claim 1, wherein 4) the wet-ground slurry isfiltered.
 3. The slurry as claimed in claim 1 or 2, wherein 5) thevolume of solvent removed is replaced in whole or in part by water afterstep (2).
 4. The slurry as claimed in any of claims 1 to 3, having anion-forming group content of from 0.05 to 1 meq/g and a neutralizingagent content of from 0.05 to 1 meq/g.
 5. The slurry as claimed in anyof claims 1 to 4, having a viscosity of (i) from 50 to 1000 mPas at ashear rate of 1000 s⁻¹ and (ii) from 150 to 8000 mPas at a shear rate of100 s⁻¹.
 6. The slurry as claimed in any of claims 1 to 5, having asolids content of from 10 to 60% by weight, in particular from 20 to 50%by weight.
 7. The slurry as claimed in any of claims 1 to 6, wherein theaverage size of the particles is from 2 to 6 μm.
 8. The slurry asclaimed in any of claims 1 to 7, comprising ionic thickeners andnonionic associative thickeners.
 9. The slurry as claimed in any ofclaims 1 to 8, wherein the particles comprise polyols as binders andblocked polyisocyanates and/or tris(alkoxycarbonylamino)triazines ascrosslinking agents.
 10. The slurry as claimed in claim 9, wherein theparticles comprise polyacrylates as binders and blocked polyisocyanatesas crosslinking agents.
 11. The slurry as claimed in any of claims 1 to10, wherein the particles have a minimum film-forming temperature of atleast 0° C., in particular at least 10° C.
 12. The slurry as claimed inany of claims 1 to 11, wherein the organic solvents are removed attemperatures below the minimum film-forming temperature of theparticles.
 13. The slurry as claimed in any of claims 1 to 12, which isfree from external emulsifiers.
 14. The use of the powder clearcoatslurry as claimed in any of claims 1 to 13 to produce clearcoats forautomotive OEM finishing and automotive refinish, for furniture coatingand for industrial coating, including coil coating, container coatingand the coating of electrical components.